When a computer shuts down by itself is generally due to several factors:
1-Overheating
2-Overclocking
3-Power issues
4-Hardware defect
5-Sometime driver issues
6-Incompatible hardware (RAM as an example).
Those above are some of the general reasons for a computer to shut down.
When you upgraded from the R5 1600 to the R7 5800X did you change the CPU Cooler that came bundled with the 1600 - Wraith Spire?
If you are using the Wraith Spire that came bundled with the 1600 on the 5800X it probably is overheating.
The 1600 TDP rating is 65 watts while the 5800X TDP Rating is 105 Watts.
While the Maximum Operating Temperature of the 1600 is 95c the 5800X is 90c.
You will need to upgrade the CPU Cooler to one that has a minimum TDP Rating of at least 150 Watts.
To eliminate Overheating and power issues run OCCT CPU, GPU, and PSU Stress tests and keep a close eye on Temperatures, Fan Speeds and PSU Outputs during all tests. See if the CPU test passes or fails while running.
Also when you upgraded from the 1600 to 5800X are you using the same RAM Memory as before?
The 5800X might not be compatible with the 1600 RAM Memory. So you will need to look at your Motherboard's QVL List for RAM MEMORY for the 5000 series processors or the RAM Manufacturer's QVL List or AMD RAM Compatibility list for the 3000 - 5000 series processors.
